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Concrete Construction

Placing concrete involves mixing, transporting, placing, consolidating (rodding or vibration to remove entrapped air), and finishing. Finishing operations in order are screeding (striking off to grade with a straightedge), floating (embedding aggregate, leveling), and troweling (smooth, dense surface). After placement, concrete must be cured (kept moist) to develop strength.

Two key controls: the slump test measures consistency/workability, and the water-cement (w/c) ratio governs strength — lower w/c gives higher strength (Abrams' law). Standard strength is the 28-day compressive strength of a cylinder, $f'_c=P/A$. During placement, fresh concrete pushes on formwork with a near-hydrostatic lateral pressure.

$$f'_c=\frac{P}{A}, \qquad \frac{w}{c}=\frac{\text{mass of water}}{\text{mass of cement}}, \qquad p_{\text{form}}=\gamma_c\,h$$

★ Consistency of Fresh Concrete

Which standard field test measures the relative consistency (workability) of fresh concrete, and what does a higher reading indicate?

The slump test measures consistency. Fresh concrete is placed in a standard cone in 3 layers, rodded, and the cone is lifted; the drop in height is the slump.

A higher slump means a wetter, more workable mix. Typical slumps: 25–50 mm for pavements/footings, 75–100 mm for beams and columns. Final answer: slump test.

★ Compressive Strength of a Cylinder

A standard 150 mm diameter concrete cylinder fails under an axial load of 320 kN. Determine its compressive strength.

$$A=\frac{\pi}{4}(150)^2=17{,}671\ \text{mm}^2$$
$$f'_c=\frac{320{,}000}{17{,}671}=18.1\ \text{MPa}$$

Final answer: 18.1 MPa.

★★ Water-Cement Ratio

A batch contains 350 kg of cement and 175 kg of water. Determine the water-cement ratio and comment on the expected strength.

$$\frac{w}{c}=\frac{175}{350}=0.50$$

A w/c of 0.50 is typical for normal structural concrete (~21–28 MPa). Lowering it increases strength but reduces workability. Final answer: w/c = 0.50.

★★ Concrete Yield from Cement Bags

A pour uses 72 bags of cement for a Class A mix that requires 9 bags of cement per cubic meter. What volume of concrete is produced?

$$V=\frac{72\ \text{bags}}{9\ \text{bags/m}^3}=8\ \text{m}^3$$

Final answer: 8 m³ of concrete.

★★★ Lateral Pressure on Wall Formwork

Fresh concrete with a unit weight of 23.6 kN/m³ is placed in a wall form to a height of 2.5 m. Assuming full liquid head, determine the maximum lateral pressure at the base and the resultant force per meter width of form.

Treat fresh concrete as a liquid (conservative). Pressure varies linearly with depth.

$$p_{\max}=\gamma_c\,h=23.6(2.5)=59.0\ \text{kPa}$$
$$R=\tfrac{1}{2}\,p_{\max}\,h=\tfrac{1}{2}(59.0)(2.5)=73.75\ \text{kN/m}$$

The resultant acts at $h/3 = 0.833$ m above the base. Final answer: 59.0 kPa, 73.8 kN/m.

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Exam Generator Problems

Additional board-style practice items for this topic.

Question Bank: w99

MSTE - Construction / Concrete Construction / MSTE November 2019

To level a floor or layer of concrete with a straight edge using a back-and-forth motion while moving across the surface.

  1. floating
  2. trowelling
  3. finishing
  4. screeding
Screeding levels fresh concrete by drawing a straight edge (screed board) across the surface in a back-and-forth (sawing) motion, distributing material evenly and removing high and low spots.
Floating, trowelling, and finishing are later operations that smooth and densify the already-screeded surface.
$\boxed{\text{screeding}}$

Question Bank: w103

MSTE - Construction / Concrete Construction / MSTE November 2019

A test to determine the relative consistency of concrete.

  1. water-cement ratio
  2. aggregate-cement ratio
  3. all of these
  4. slump test
The slump test measures the relative consistency or workability of fresh concrete. A slump cone is filled in three layers, each rodded, then lifted, and the settlement (slump) is measured in millimeters.
$\boxed{\text{slump test}}$